Unix uses a hierarchical file system for storing files. This system
consists of a "root" directory which is made up of sub-directories
which in turn are made up of sub-directories.
If you are not already familiar with the concept of a hierarchical
file system, see one of the general books listed in the references
for a detailed discussion of this structure.
ls Command
The ls command is used to list the contents (names of files) in a
directory. If no
directory path
is specified,
ls will display the files in the current directory.
Display a list of files:
ls
Display file characteristics:
ls -F
When this option is used,
the names of
executable files are followed by an asterisk ("*") and
directory names will be followed by a slash ("/").
On some Unix systems, directory names are followed by both a slash and
an asterisk ("/*").
Files which begin with a dot (.) are hidden and not
displayed by the ls command unless specifically requested. These files
are frequently used as configuration files by application programs.
To display these files, enter:
ls -a
To obtain a "long" listing of all files in the current directory, enter:
ls -la
This listing includes the following information:
-
File Permissions
- who has permission to read, write, or execute the
file.
- a "d" in the first column indicates that this file corresponds to a
directory, a "-" indicates a regular file.
- the next three columns indicate whether the owner has read, write,
or execute permission for the file.
- the next three columns indicate whether other members in your group
have read, write,
or execute permission for the file.
- the next three columns indicate whether individuals other than
the owner and members in the owners group
have read, write,
or execute permission for the file.
- Number of Links
-
- Owner Name
-
- Group Name
- on some Unix systems (typically BSD based)
it is necessary to include the
-g flag in order to display the group name; on VTAIX and System V
based systems, inclusion of the -g option will suppress display of
the group membership column.
- Size in Bytes
-
- Date of Last Modification
-
- Filename
-
Directory Utilities
The primary commands used when working with directories are:
cd
pwd
cd xmp
pwd
mkdir make directory
mkdir test_dir
ls -F
mv move files between directories
ls *.c
ls test_dir
mv my_source.c test_dir
ls *.c
ls test_dir
rmdir remove directory
ls -a test_dir
rmdir test_dir
mv testdir/mysource.c .
ls -a test_dir
rmdir test_dir
ls -F
Note: You cannot remove a directory until you have first
moved or removed (deleted) all of
the files it contains. You must also have write permission for the
parent directory containing the directory you wish to remove.
Directories & System Organization
You should become familiar with the definitions of the following
terms:
- Current Working Directory (.)
- the
directory within which you are currently working.
In an earlier exercise, you copied the sample files to the "."
directory. To Print (display) the Working Directory, enter:
pwd
- Parent Directory (..)
- the directory immediately
above the current directory in the directory hierarchy.
Observe the
results of entering the following commands:
pwd
cd ..
pwd
cd ..
pwd
- Home Directory
- the default directory
associate with a userid.
To change to the home directory of the VTAIX Class Instructors Id, enter:
cd ~aixstu00
The ~ abbreviation is only available if you are using the
C or Korn Shell.
To change to your own home directory, you can simply enter:
cd
- relative path
- start from current directory (.)
Observe the results of the following commands:
cd
cd xmp
pwd
cd c
pwd
ls
- absolute path
- start from "root" (/)
Observe the results of entering the following commands:
cd /etc
cd xmp
cd $HOME/xmp
ls
cd /
ls
cd
ls
Executing Commands
Include ./ in front of the filename.
Enter the filename (use "echo $PATH" to display the
search path).
Include absolute path followed by filename.
The Typical Unix File System
As an exercise, change to the root directory and observe the
sub-directories it contains:
cd /
ls -F
Recall that the "-F" option of the ls command appends a slash ("/")
to the end of sub-directory names.
You should see most of the following directories, plus some others:
Note: On VTAIX and BSD based Unix systems, the symbol @ is used to
indicate a symbolic link. Observe here that
"bin" is a symbolic link and
displayed as "bin@" instead of "bin/" as it would be on many other
Unix systems.
Enter the command "ls -lF b*" and observe the
"->" following "bin@".
This symbol points to the directory /usr/bin which
contains the files which are accessed whenever a reference is made to the
/bin directory.
The home directories for each of the userids on the system
may be included in directories called "us1", "us2", and "us3".
On other Unix systems, these directories may be called
home, users, or u.
To return to your home directory, simply enter:
cd
Exercise: Using the Directory Utilities
Observe the results of entering the following commands:
pwd
cd
cp xmp/letter memo
mkdir my_test
cd my_test
cp ../xmp/cars .
ls -la
cd ~aixstu00
ls bin
ls /bin
pwd
ls
rm memo
You are not able to remove this file as neither group nor other
have "write" permission in the home directory of aixstu00. See the
discussion of access permissions in the next section.
cd
rm memo
rmdir my_test
You are not yet able to remove this directory as it is not empty. First
you must either remove or move the files it contains to another location.
ls -a my_test
rm my_test/*
rmdir my_test
File Access Permissions
Recall
that you can display the file access permissions by entering:
ls -l
At the beginning of each entry you will see 10 columns containing
information about file characters.
d r w x r - x r - -
The first column provides information about the file type: "d"
for a directory, "l" for a symbolic link, or "-" (dash)
for a regular file.
The next columns contain the
permission fields for
user (owner), group, and other:
- r
- read
- w
- write
- x
- execute
The "read", "write", and "execute" permissions associated with a
file indicate whether you can look at the contents of the file,
make changes to the file, or execute the program contained in the file.
You can change the current access permissions using the chmod command.
The general syntax of the chmod command is given by:
chmod ugoa±rwx file_name
where:
(u=user or owner, g=group, o=other, a=all)
(r=read, w=write, x=execute)
For example, to
add execute permission for owner to the file
"file_name", enter:
chmod u+x file_name
You can specify more than one option with the chmod command; for
example, to remove read and execute permission for group and other from
a file called file_name, enter:
chmod og-rx file_name
Be careful how you assign access permissions. It is possible to
deny yourself read, write, and execute permission while granting
these permissions to group and other.
You can also use numeric codes to
change the current access permissions for a file.
A chmod value of 1 enables execute permission,
a value of 2 enables write permission, and a value of 4 enables
read permission. Combinations of read, write, and execute
can be obtained by adding the values corresponding to the permissions
you wish to assign. For example, read and execute permission can be
specified by adding the values 4 and 1 to yield 5.
These codes are summarized in the following table:
1 --x execute only
2 -w- write only
3 -wx write and execute
4 r-- read only
5 r-x read and execute
6 rw- read and write
7 rwx read, write, and execute
To change
the access permissions of "file_name"
to read, write, and execute for owner (4+2+1=7),
with read and execute for group (4+1=5),
and execute (1) only for other, enter:
chmod 751 file_name
Then observe the results of the following commands:
ls -l file_name
./file_name
cat file_name
chmod a-rx file_name
ls -l f*
chmod a+r file_name
ls -l f*
Note: As illustrated above, it may be possible to turn on execute
permission for a file; however, if the file does not contain meaningful
commands, you will obtain nothing but error diagnostics when you
attempt to execute it.
The umask command sets a mask which limits the access permissions
assigned to new files.
To display the current umask values, simply enter:
umask
The "umask" values are complements of the
chmod values: a 1 denies execute permission, a 2 denies write access,
and a 4 denies read access. As with the chmod command, the "umask"
values can be combined; e.g., read-only access (denial of execute and
write permission can be specified by adding 1 and 2 to yield 3).
A seven (7) denies all access permissions.
To set the umask values to allow the owner
read, write, and execute (0 limits) permission for all new files;
to deny group write (2) access (which is equivalent to allowing
read and execute permission), and to deny other read and write (2+4=6)
permissions (which is equivalent to granting execute only), enter:
umask 026
Note: The value 026 is the complement of 751 (777-026=751) used in the
chmod example described in the previous section.
To set the default permission mask to allow the owner read, write, and
execute capabilities and to limit group and other execute and read
access, enter:
umask 022
To set the umask
for future logon sessions, include this command in the
file ".profile" (Bourne or Korn Shell) or ".login" (C Shell).
The following table lists some of the most commonly used umask values.
In each of these cases, the
owner is assigned default read, write,
and execute permissions; group and other are assigned the default
access permissions listed in the
column labeled "group & others":
chmod umask group & others
777 000 read, write, & execute
755 022 read & execute
744 033 read only
711 066 execute
700 077 no access
The values given in the "umask" column can be obtained by
subtracting the "chmod" values from "777".
The access permissions associated with directories
influence the operations which can be performed on
files they contain.
The meanings of the directory access permissions are:
- read
- allows display of a directory -- read permission is required
in order to use ls to list characteristics of files within a directory;
otherwise you will receive the message "No permission" following the
name of each file in the directory.
- execute
- enables access to the contents of a directory -- execute
permission is required in order to cd into a directory or to access
(execute or examine) any of the files it contains. Without execute
permission, you will typically receive the message "Permission Denied".
- write
- enables creation, deletion, and moving of files within the
directory.
Thus deletion of a file is limited by the permission
associated with the directory and not by the individual file access
permissions. Be careful to whom you provide directory write
access permissions. They will not be allowed to modify the file
if they do not have file write access permission, but they can remove it
if they have directory write permission.
If you wish to share files with others, you need to be sure that
both the permissions for the file and the directory containing it are
set appropriately.
Changing Group Membership
It is possible for your userid to belong to more than one group
(the list of groups and their memberships is typically
contained in the file /etc/groups).
On VTAIX, your userid will be listed in /etc/groups only
if your userid belongs to more than one group.
Use the "ls -l" command ("ls -gl" on some systems) to display
a list of files and their group membership.
If you would like to change the group to which an existing
file is assigned, use the "chgrp" command.
If you would like to change the default group used for the assignment
of new files, use the "newgroup" command.
Note: On most Unix systems, you will only be able to use the chgrp and
newgroup commands for groups of which you are a member.
i-nodes
The internal "name" used by the system for each
file is its i-node number.
Each directory in a Unix system is a file which lists
the files it contains with their corresponding i-nodes.
An i-node is a data structure which defines a file's existence and
contains information about
its location on disk, size, ownership, permissions,
and time of last modification.
To display the i-node number corresponding to file_name, enter:
ls -i file_name
Observe the result of the following commands:
ls -i page2
mv page2 new_name
ls -i new_name
Although the mv command has changed the name of the file, the i-node
number remains unchanged.
Locating Information
The find
command can be used to locate a file or group of files.
To list
all files beginning in the current directory down through
the directory tree which begin with the letter 'f':
find . -name 'f*' -print
The find command can also be used to display directory listings by
using the "-type" option. For
example, to display all directories below the current directory, enter:
find . -type d -print
The whereis command is used to locate the binary,
source, and
manuals for a command. To locate the occurrence of "vi" in any
of the standard locations, enter:
whereis vi
If an executable program is located in more than one directory, you may
wish to know which of these files will be executed. To determine
this, you can use the
which command.
For example, to identify the directory
containing the command which will be issued when "vi" is entered, enter:
which vi
If a command
alias
has been defined,
the which command will display the alias for the command.
The grep command (Global Regular Expression Print) can
be
used to locate text within files. For example, to list all files in the
xmp/c directory containing the word 'float', enter:
cd xmp/c
grep 'float' *
To print a list of all lines in the file /etc/passwd containing the
name of your userid, enter:
grep $USER /etc/passwd
Note: The above command will not yield the same result on
all Unix systems; some Unix systems (such as NeXT) store their userid
information in other locations.
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